1. Field of the Invention
The present invention relates to a technique for manufacturing high strength cold-rolled steel sheets or hot-dip galvanized steel sheets primarily used for automotive panels or structural components, and, more particularly, to cold-rolled steel sheets and hot-dip galvanized steel sheets having good wettability while guaranteeing mechanical properties including a tensile strength of 590 MPa or more and a strength-ductility balance (TS×El) of 16,520 MPa·% or more, and a manufacturing method thereof.
2. Description of the Related Art
In recent years, automobile manufacturers have made various attempts to increase strength of a vehicle body and enhance fuel efficiency in order to satisfy provisions relating to increasingly strengthened safety and environmental regulations. Automobile manufacturers have made efforts towards development of automobiles that are environmentally friendly while having high strength and reduced weight.
Further, with complicated designs of automobiles and diversification of consumer demand, the automobile manufacturers also require steels that have high strength with good workability and formability.
Since strengthening steel sheets for automobiles leads to deterioration in formability, however, it is difficult to satisfy both strength and formability at the same time. In addition, impurities added for strengthening the steel sheets make it more difficult to manufacture a plated steel sheet with a pleasant surface.
For automotive interior panels, it has been attempted to achieve high strength by enhancing formability of the existing phosphorous (P)-added high strength steel, but desired strength is still not obtained due to a strength reduction resulting from insufficient formability and thickness reduction. For the automobile manufacturers, however, since it is possible to achieve cost reduction through reduction of the number of processes by application of high strength steel with good formability, the development of high formability and high strength steel has been consistently required.
For automotive exterior panels, soft cold-rolled steel sheets, for example, extremely low-carbon IF (interstitial-free) steel, and 340 MPa-grade high formability and high strength steel are primarily applied, and higher strength steel sheets are applied to some automotive components, which require higher strength.
In order to enhance strength and formability of such a steel sheet for automobiles, solid solution strengthening elements, such as silicon (Si), manganese (Mn), phosphorous (P), and the like, are generally added to improve strength of the steel sheet, and carbon nitride formation elements, such as titanium (Ti), niobium (Nb), and the like, are generally added to enhance formability. For example, multi-phase high strength steel sheets have been developed.
The multi-phase high strength steel sheet has a combined soft ferrite structure and hard martensite structure and demonstrates low yield strength and high strength-ductility balance.
However, silicon (Si), manganese (Mn), and the like added for strength enhancement cause concentration of silicon-based oxides on the surface of the steel sheet during annealing after cold rolling, so that surface characteristics of the plated steel sheet are deteriorated, thereby making it difficult to manufacture galvanized steel sheets with pleasant surfaces for automotive applications.
As hot-dip galvanized high strength steel sheets with good formability, a steel sheet has been suggested, which comprises, in % by weight (hereinafter, wt %), C: 0.12˜0.70%, Si: 0.4˜4.8%, Mn: 0.2˜2.5%, Al: 0.01˜0.07%, N: 0.02% or less, and the balance of Fe and unavoidable impurities. This steel sheet is based on so-called Transformation Induced Plasticity (TRIP), and has a combined structure of ferrite, bainite and residual austenite.
As compared with a steel sheet having a multi-phase of ferrite and martensite and the same strength, this steel sheet has a much higher Si content of 0.4 wt % or more, which leads to deterioration in paintability and wettability, thereby making it difficult to produce galvanized steel sheets with pleasant surfaces.
Therefore, a long-term pickling process is required to guarantee desired paintability and wettability with TRIP steel sheets, thereby causing an increase of manufacturing costs.
In recent years, as a steel sheet capable of satisfying both good formability and high strength after formation, a bake hardening (BH) steel sheet has been developed, which is soft before pressing to allow easy pressing and is hardened by paint phosphating after pressing, thereby providing high strength to components.
One example of the BH steel sheet includes a high strength cold-rolled steel sheet, which comprises, in % by weight, C: 0.05˜0.30%, Si: 0.4˜2.0%, Mn: 0.7˜3.0%, Al: 0.02% or less, N: 0.0050˜0.0250% and dissolved N: 0.0010%, has a combined structure of ferrite, bainite and residual austenite, and exhibits good age hardening properties.
However, this steel sheet also has an Si content of 0.4 wt % or more in order to stabilize the residual austenite, which leads to deterioration in paintability and wettability, thereby making it difficult to produce galvanized steel sheets with pleasant surfaces.
In other words, conventionally, a large amount of Si greater than or equal to 0.4 wt % is added to the steel sheet in order to form the combined structure consisting of the ferrite, bainite and residual austenite while significantly enhancing tensile strength and strength-ductility balance. This is because carbon required for generation and stabilization of the residual austenite can be effectively concentrated in austenite during annealing by addition of a great amount of Si which serves to suppress formation of Fe3C.
The steel sheet containing Si in an amount of 0.4 wt % or more has enhanced tensile strength and strength-ductility balance, but suffers from concentration of silicon-based oxides on the surface thereof, which leads to deterioration of paintability and wettability, thereby making it difficult to produce galvanized steel sheets with pleasant surfaces.